A long-standing programmatic tension within the Supplemental Nutrition Program for Women, Infants and Children (WIC) has been the promotion of breastfeeding coupled with the availability of free infant formula. Breastfeeding initiation and duration is substantially lower among WIC participants than their eligible non-participants. Peer counseling has been viewed as a potentially cost-effective means of increasing breastfeeding without removing the safety net of free formula. In fact, the recent Surgeon General's Call to Action on Breastfeeding recommends that peer counseling become a core service in all WIC agencies. The proposed study will evaluate the effectiveness of a telephone-based peer counseling program to improve breastfeeding among WIC recipients. The study leverages already collected data from the largest randomized control trial (RCT) of peer counseling for breastfeeding ever undertaken among WIC clients by contrasting results from the RCT conducted in four counties in Oregon to those from a non-randomized peer counseling intervention of the same design that was implemented at the same time in two different counties in the state. As a third contrast, we will analyze changes in breastfeeding among WIC clients in the same state over the same study period but in counties that had no peer counseling intervention. Data from the last two sources will be extracted from Oregon's WIC administrative data system. We will compare estimates of the intention-to-treat as well the effect of treatment on the treated in the RCT to those obtained from a non-randomized design. This will provide the first estimates of selection bias associated with a peer counseling intervention based on observational data. In addition, we can estimate the potential external validity of results from th RCT by contrasting the characteristics and breastfeeding outcomes of WIC participants in the control arm of the RCT to those of WIC participants in counties with no peer counseling. By exploiting data in three distinct settings, (RCT of peer counseling, non-RCT of peer counseling and no peer counseling), we will provide a comprehensive examination of the potential of peer counseling to enhance the initiation, duration and exclusivity of breastfeeding among WIC participants.